For a twin turbo 2jz, you are looking at an easy $2500 just for the motor and wiring harness.  It is a sought after engine in the MKIII Supra world.  The twin turbo is an easy 300hp, and 400hp w/basic power mods.  IMO, a lot more power/torque than is need for a trail rig.  The NA version is rated at 220, but isn't cheap to buy either.  The turbo wouldn't get too hot on the trail, unless you were spooling the turbo.  You would have to use a turbo boost adjuster to zero pounds to prevent the turbo's from spooling when crawling.  The real issue w/the turbos on a trail rig would be when using them you have to let the engine idle a few minutes after use to cool down, which on teh trail often isn't that easy to do w/a trail rig. 

If you want a straight 6, you should look into a 5m or 7m.  The 5m is about 161hp, and the 7m(non turbo) is about 200hp, both of which can be built for more power.  Both of them should be cheap to get, since Supra builders often dump them for turbo'd 7m's, 1.5jz's, or 2jz's. 

Another option would be to do a power build of a 22r(e), which should be good for about 145 and would be a lot cheaper/easier than any swap.  A buddy of mine ran a built-up 22r in his '84 w/a Webber, it did well in the mountains, and he was running 36's w/5.29's.